1. Field of the Invention
The present invention relates to an electronic apparatus in which a fuel cell is used as a power supply, and to a fuel cell system.
2. Description of the Related Art
Electronic apparatuses, such as a portable audio device and a personal digital assistant (PDA), have recently been remarkably reduced in size. With the reduction in size of these electronic apparatuses, there has been an attempt to use a fuel cell as a power supply. The fuel cell has the advantage that electricity can be generated simply by supplying a fuel and an oxidizer and electricity can continuously be generated only by replacing the fuel. Thus, if the size of the fuel cell can be reduced, the fuel cell is very effective as a power supply of a small-sized electronic apparatus.
Attention has recently been paid to a direct methanol fuel cell (DMFC) as a fuel cell. The DMFC is configured such that an electrolyte membrane is disposed between an anode and a cathode. Each of the anode and cathode comprises a collector and a catalyst layer. At the anode, methanol aqueous solution is supplied as fuel, and protons are generated by catalysis. On the other hand, at the cathode (air electrode), air is supplied from an air intake port. At the cathode, protons, which have passed through the electrolyte membrane, react with oxygen, which is contained in the air that is supplied from the outside, on the catalyst, and thereby electrical power is generated. In this manner, in the DMFC, methanol having high energy density is used as fuel, and an electric current is directly obtained from the methanol on the electrode catalyst. Since the methanol requires no reforming, the DMFC can be reduced in size and the handling of fuel is easier, compared to hydrogen gas. Thus, the DMFC is prospective as a power supply of a portable electronic apparatus.
On the other hand, in the electronic apparatus using the fuel cell as a power supply, a secondary cell is used as an auxiliary power supply for supplying auxiliary power in compensation for deficiency of power that is output from the fuel cell. The secondary cell is always charged by the power that is output from the fuel cell.
Jpn. Pat. Appln. KOKAI Publication No. 2000-106201, for instance, discloses such a fuel cell. In addition, PCT International Publication No. WO2005/043664 discloses a fuel cell system which enables power supply to an electronic apparatus.
As regards these conventional electronic apparatuses, it is known that the time in which the electronic apparatus is not used is, in most cases, longer than the time in which the electronic apparatus is actually used. However, even in the state in which the electronic apparatus is not used, that is, even in the state in which the load side does not operate at all, fuel is constantly supplied to the fuel cell, and output power is generated from the fuel cell. Thus, the secondary cell is being charged by the fuel cell in the state in which the electronic apparatus is not used. In other words, the secondary cell is continuously charged by the fuel cell in a very low load state in which the electronic apparatus is not used. Consequently, there arises such a problem that the secondary cell is kept for a long time in a fully charged state or in an approximately fully charged state.